Transport Scotland announced that the £40-£60 million contract for dualling more than 26 miles of the stretch of the A9 between Glengarry and Dalraddy is going to CH2MHill/Fairhurst.
The designers will start work on the middle section of the route next month, subject to the "mandatory standstill period" which gives other bidding parties the opportunity to lodge any objections.
Three sections of the A9 between Perth and Inverness are now expected to be ''shovel ready'' for work to begin in 2017.
Most of the notoriously dangerous road is single carriageway, with the Scottish Government already having pledged to upgrade it to dual carriageway by 2025, with a £3 billion project to convert 80 miles of roadway.
Transport Minister Keith Brown said: "The A9 dualling programme is the most expensive transport project in Scotland's history and presents some of the most challenging work ever witnessed.
"The appointment of the first design consultancy for this mammoth project marks another significant milestone in this Government's commitment to the project - the first Government to commit to dual this vital route between Perth and Inverness.
"As well as delivering faster and more reliable journey times and road safety improvements when complete, the design work offers the prospect of steady work for many years ahead for the three successful contractors.
"There will also be many opportunities for small and medium enterprises through sub-consultancy work and the winning bidder has also committed to providing 35 full-time roles and 28 summer placement opportunities for apprentices, work experience students and graduates, bringing much-needed opportunities for young people to gain experience working on this vital project linking central Scotland to the Highlands and Islands."
Three design contracts will be awarded for the dualling programme, each valued at £40-£60 million.
Mr Brown also announced that subject to the mandatory standstill period, Transport Scotland will award the ground investigation contract for the four-mile Birnam to Tay Crossing section to Soil Engineering.
Work is expected to begin next month and last for six months.
He said: "There is no let-up in the work going on and these important ground investigations will give us the detailed information needed to progress the draft Orders for the difficult Birnam to Tay Crossing section, expected to be published in 2015.
"Like much of the route, this section is very constrained and we are working to minimise any impacts to properties, the railway and the environment."